Six environmental activists who faced charges of trying to take over a power station called today for an inquiry into the role of an undercover police officer, who is accused of helping to plan and pay for the invasion.

The trial of the six, who denied conspiring to break into Ratcliffe-on-Soar power station, collapsed today, after details began to emerge about the role played by PC Mark Kennedy, an undercover police officer.

The home secretary, Theresa May, has been called to give a statement to parliament after evidence emerged suggesting he turned agent provocateur. Kennedy, who has quit the Metropolitan police force and is living abroad, has told friends he regrets the operation, which he described as "wrong".

During a short hearing at Nottingham crown court, prosecutor Felicity Gerry told the judge that the Crown Prosecution Service had conducted a review of the case and decided not to submit evidence.

Judge John Milmo agreed to enter not guilty verdicts against all six defendants, who were among 114 activists arrested in April 2009, on the eve of a protest in which some planned to occupy the power station.

The six activists, who were charged with conspiracy to commit aggravated trespass, acknowledged they had gathered at a school in advance of the protest, but denied they were among those who had committed to breaking at the power station.

Speaking after the hearing, their solicitor Mike Schwarz, confirmed Kennedy offered to assist the defence during the trial, saying he believed the police officer turned "native" after being convinced of the arguments of climate change.

"On Easter Monday 2009, over 400 police officers were involved in a raid at Iona school in Nottingham, which led to 114 arrests. I represented 113 of those arrested," Schwarz said. "The 114th we now know was PC Kennedy, an undercover police officer."

He added that his clients had not agreed to join in the planned occupation of the power station. "The evidence of PC Kennedy presumably confirmed this. Yet that evidence, had it been kept secret, could have led to a miscarriage of justice.

"Serious questions must be asked relating to the policing of protest, from the use of undercover officers, to the use of expensive and legally questionable mass pre-emptive arrest of protesters, to extremely restrictive pre-charge bail conditions, to the seemingly arbitrary nature by which the 114 initially arrested were reduced to the final 26 who were eventually charged."

Schwarz added: "The police need to answer some serious questions about their conduct relating to protesters."

"This is a serious attack on peaceful, accountable protest on issues of public and pressing importance like climate change."

"One expects there to be undercover police on serious operations to investigate serious crime. This was quite the opposite," he said.

The Met has declined to comment, saying: "We are not prepared to discuss the matter." The Association of Chief Police Officers, which runs the national "domestic extremism" agency that Kennedy reported to, has also declined to comment. A spokesperson for that agency - the National Public Order Intelligence Unit - said: "We don't comment on these matters."

The activists were given lenient sentences last week, after a judge recognised the protest, which would have caused minimal damage, was intended as a "legitimate" demonstration by individuals with "the highest possible motives".

The Guardian's investigation into Kennedy, who used a fake ID to infiltrate protest groups in 22 different countries, prompted concern in Westminster today. David Winnick, a Labour MP and member of the home affairs select committee, said he was most concerned over Kennedy's alleged shift from passive observe to agitator, and called on the home secretary to make a statement to parliament.

"The concern is not the fact that the Metropolitan police, and possibly other police forces, use undercover agents," he said. "No one is so naive as to believe that that hasn't been the case since time began.

"My concern is the manner in which it has been alleged that Kennedy acted almost as an agent provocateur. In these circumstances, I think Mrs May should come to the Commons and make a statement."

Kennedy's identity was only found out in October, when friends who had grown suspicious of his behaviour discovered a passport bearing his real name. Confronted with other documents that proved he had been a police officer since 1994, Kennedy confessed to them that he was working undercover.